chore(neptune): add engine version 1.4.5.1 (aws#35362)

### Issue # (if applicable)

None

### Reason for this change

Neptune had released engine version 1.4.5.1.

- Docs
  - https://docs.aws.amazon.com/neptune/latest/userguide/engine-releases-1.4.5.1.html
- Version confirmed via CLI
  - `aws neptune describe-db-engine-versions --engine neptune --engine-version 1.4.5.1 --region us-east-1
{
    "DBEngineVersions": [
        {
            "Engine": "neptune",
            "EngineVersion": "1.4.5.1",
            "DBParameterGroupFamily": "neptune1.4",
            "DBEngineDescription": "neptune",
            "DBEngineVersionDescription": "Neptune 1.4.5.1.R1",
            "ValidUpgradeTarget": [],
            "ExportableLogTypes": [
                "audit",
                "slowquery"
            ],
            "SupportsLogExportsToCloudwatchLogs": true,
            "SupportsReadReplica": false,
            "SupportsGlobalDatabases": true
        }
    ]
}`

### Description of changes

Added `V1_4_5_1` to EngineVersion class


### Description of how you validated changes

None

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: matoom-nomu

packages/@aws-cdk/aws-neptune-alpha/lib/cluster.ts

@@ -134,6 +134,10 @@ export class EngineVersion {
    * Neptune engine version 1.4.5.0
    */
   public static readonly V1_4_5_0 = new EngineVersion('1.4.5.0');
+  /**
+   * Neptune engine version 1.4.5.1
+   */
+  public static readonly V1_4_5_1 = new EngineVersion('1.4.5.1');
 
   /**
    * Constructor for specifying a custom engine version